Use Google Books to search across the full text of books (those included in the database in full text) to find citations within books. Google Books should return a 'preview' of a book page where a citation appears, even if the book is not available on Google in full text. This works best when you are looking for citations of a book or article with a unique title, or an author with a unique name.
Google Scholar is useful for locating citations in books and journals. Search for the title of the book or book chapter to locate citations. PhD and Masters theses can be considered scholarly citations as well These will generally be housed in university Institutional Repositories - many of which are crawled by Google Scholar.
JSTOR provides electronic full text of complete back sets of over a thousand major journals in a range of disciplines, including social sciences, business and economics.